Brit head over heels for messy hairdo (3)
I believe the answer is:
mop
'messy hairdo' is the definition.
(I've seen this in another clue)
'brit head over heels' is the wordplay.
'brit' becomes 'pom' (pom is a kind of brit).
'head over heels' is a reversal indicator (in a down clue, letters flip upside down).
'pom' in reverse letter order is 'MOP'.
'for' is the link.
(Other definitions for mop that I've seen before include "Polish" , "Swab" , "Clean; mass of hair" , "Instrument for cleaning" , "Floor cleaner" .)